Jeremy Corbyn under pressure to back cross-party vote to stay in the single market and customs union

Britain's Green Party MP Caroline Lucas speaks during the party's general election campaign launch in central London February 24, 2015. Britain will go to the polls in a national election on May 7. REUTERS/Stefan Wermuth

Labour urged to back Report Stage amendment which would block Theresa May from taking Britain out of the single market and customs union.

Lib Dem, SNP, Plaid Cymru and Green Party leaders are tabling the amendment.

Corbyn repeated his disputed claim that single market membership relies on EU membership in an interview with Robert Peston on Sunday.

LONDON — The Liberal Democrats, SNP, Plaid Cymru, and Green Party have urged Labour MPs to back an amendment blocking Theresa May from dragging Britain out of the single market and customs union.

The Westminster leaders of these opposition parties plan to table a Report Stage amendment to the EU Withdrawal Bill which, if passed, would stop the government from removing Britain from the two institutions as part of Brexit.

This follows a meeting of opposition parties earlier this month in which leaders agreed to do all they can to prevent a hard Brexit, which they see as Britain quitting the single market and customs union as it departs the EU.

Labour leader Jeremy Corbyn refused to attend the meeting.

However, the other opposition parties hope the amendment will attract support from enough Labour MPs to put the government at risk of a significant defeat.

"This amendment is a significant step in the fight against Brexit, demonstrating the increased willingness of parties to work together formally rather than just behind the scenes," Lib Dem leader Sir Vince Cable said.

"Through the four leaders in the House of Commons, we are presenting a united front.

"We urge the leadership of a fifth opposition party, Labour, join us so we can change course from the dangerous Brexit being pursued by the Conservatives."

"It's heartening to see parties working together to halt the calamitous Tory Brexit plans," Green Party joint-leader Caroline Lucas added.

"What happens this year in Parliament will have huge and lasting repercussions for the country — which is why we're putting party differences aside and working alongside each other.

"We urge the Labour leadership to join us in backing this crucial amendment — and show that they too are willing to put the national interest first."

Labour divided over the single market

Corbyn is coming under increasing pressure from numerous Labour MPs to commit to keeping Britain both within the single market and customs union.

MPs such as Chuka Umunna, Stephen Kinnock and Heidi Alexander have all urged the party leadership to embrace a softer form of Brexit. 87% of party members want to stay in the single market, a recent poll showed.

Corbyn again ruled out single market membership in an interview with ITV's Robert Peston on Sunday. He said: "The single market is dependent on membership of the European Union."

Numerous Labour MPs have pointed out that Norway, Lichenstein and Iceland participate fully in the single market, despite not being EU members, through their membership of the European Free Trade Association.

Labour MP Chris Leslie tweeted:

Some MPs were left frustrated at the party's parliamentary meeting earlier this week after Corbyn reportedly ruled out adopting a Brexit policy based on staying in the single market and customs union.

Labour sources told Business Insider that Umunna and "numerous" other MPs were left disappointed.

However, Umunna did not storm out of the meeting in protest as suggested in some reports, a source close to the MP for Streatham told BI.